The present invention relates to the technical field of increasing a range of viewing angles in liquid crystal display devices and the like. More particularly, the present invention relates to a light diffusing plate having a high or an excellent light diffusing capability, a liquid crystal display apparatus and a rear projection apparatus which make use of the light diffusing plate.
The opportunity or the frequency of use of liquid crystal displays (LCDs) as a display for word processors and computers is rapidly increasing today. The use of LCDs as a monitor in ultrasonic, CT and MRI diagnostic apparatus is under review. Conventionally, these medical diagnostic apparatus have primarily used CRTs (cathode-ray-tubes) as a monitor.
LCDs have many advantages such as ease in size reduction, small thickness and lightweightness. On the other hand, they have poor viewing angle characteristics (narrow viewing angle) since as the viewing direction or angle changes, the contrast of an image degrades sharply and the gradation also reverses to have the image look differently. As a result, depending on a position of a viewer, the image can not be viewed correctly.
In medical applications described above, diagnosis with monitors is based on the difference in image density, so that not only is it required to produce an image of high contrast ratio but at the same time, incorrect or improper recognition of an image can cause wrong diagnosis or inconsistency in the results of diagnoses. Under these circumstances, it is required that images of high contrast ratio be displayed over a wide range of viewing angles, particularly, in a case that diagnosticians such as a plurality of doctors and the like view a medical diagnostic image. Another problem peculiar to medical monitors is that image is usually displayed in monochrome (black and white colors) and, hence, suffers considerable contrast degradation as the viewing angle varies.
A known method for increasing the viewing angle of LCDs relies upon using a collimated backlight and an image-bearing light that has passed through the liquid crystal display panel is diffused with a light diffusing plate (see Japanese Examined Patent Publication No. 7-7162, Unexamined Published Japanese Patent Application No. 6-95099 and the like).
In this method, it is required that the light diffusing plate not only has a satisfactory light diffusing capability but also can effectively diffuse the collimated light which has passed through the liquid crystal display panel in order to perform a high contrast image display over a wide range of viewing angles; however, such light diffusing plate has practically not been fabricated.
In the light diffusing plate used for the purpose of increasing the viewing angle in the LCD, a light shield layer (hereinafter also referred to as black mask) is ordinarily formed in an area other than an optionally-set light exit area thereof to prevent a decrease of visual recognition, an uneven display, a blurred image and the like to be caused by a stray light. On the other hand, in a conventional light diffusing plate, besides the stray light, the image-bearing collimated light which has passed through an appropriate optical path and is incident on the black mask is blocked to some extent.
As a result, the quantity of the light which passes through the light diffusing plate, is diffused and issued, that is, display luminance is decreased thereby causing cases in which the object to increase the viewing angle can not be achieved.
Moreover, in the above-described method in which the collimated light is diffused by the light diffusing plate, the more the light having high luminance and sufficiently diffused is issued, the wider the range of the viewing angles of the LCD becomes. Accordingly, the light diffusing plate used for increasing the viewing angle of the LCD is required to sufficiently diffuse the collimated light incident thereon.
The same problem of viewing angle also applies to a rear projection apparatus.
In the rear projection apparatus, an image-bearing light issued from a rear projection engine is incident on a back surface of a screen to display the image on A front surface thereof. The screen of the rear projection apparatus is ordinarily constituted of a Fresnel lens and a lenticular lens whereupon a diffused light issued from the rear projection engine is changed into a collimated light by the Fresnel lens and the resultant collimated light is diffused by the lenticular lens to display an image.
In this screen, an excellent viewing angle can be obtained in one direction, for example, in a horizontal direction,; however, a sufficient viewing angle can not be obtained in a vertical direction. It goes without saying that an image having a high luminance can preferably be displayed also in the rear projection apparatus making use of the light incident on the screen without wasting it; however, this has not been fully performed in practice.
Therefore, required is the light diffusing plate which can change the collimated light incident thereon into the diffused light which is sufficiently diffused and issue it and, further, have an excellent light diffusing capability; however, such light diffusing plate has not been put in practice.